Aromatic carbonates are useful, for example, as raw materials for the production of aromatic polycarbonates, which in recent years have become increasingly used as engineering plastics, without using toxic phosgene. As processes for the production of aromatic carbonates, a process of subjecting an alkyl carbonate and an aromatic monohydroxy compound to transesterification, a process of subjecting an alkyl aryl carbonate and an aromatic monohydroxy compound to transesterification, and a process of subjecting two molecules of the same alkyl aryl carbonate to transesterification, i.e. disproportionation, are known. Typical reactions included under processes for the production of an aromatic carbonate or a mixture of aromatic carbonates through reaction between an alkyl carbonate and an aromatic monohydroxy compound are represented by following reaction formulae (E1), (E2), (E3) and (E4):
(wherein R represents an alkyl group, and Ar represents an aromatic group. Reaction formulae (E3) and (E4) are transesterification reactions between the same molecular species, with reaction formula (E4) also generally being referred to as a disproportionation reaction).
The above reactions of (E1) to (E4) are all equilibrium reactions. For the reactions of (E1) and (E4) in particular, the equilibrium is biased markedly toward the original system, and the reaction rate is slow, and hence the aromatic carbonate yield is low, and thus there have been many difficulties in industrial production. Several proposals have been made to improve on the above difficulties, but most of these have related to development of a catalyst to increase the reaction rate. For example, Lewis acids such as transition metal halides and Lewis acid-forming compounds (see, for example, Patent Document 1: Japanese Patent Application Laid-Open No. 51-105032, Patent Document 2: Japanese Patent Application Laid-Open No. 56-123948, Patent Document 3: Japanese Patent Application Laid-Open No. 56-123949), tin compounds such as organo-tin alkoxides and organo-tin oxides (see, for example, Patent Document 4: Japanese Patent Application Laid-Open No. 54-48733, Patent Document 5: Japanese Patent Application Laid-Open No. 54-63023, Patent Document 6: Japanese Patent Application Laid-Open No. 60-169444, Patent Document 7: Japanese Patent Application Laid-Open No. 60-169445, Patent Document 8: Japanese Patent Application Laid-Open No. 62-277645, Patent Document 9: Japanese Patent Application Laid-Open No. 1-265063), salts and alkoxides of alkali metals and alkaline earth metals (see, for example, Patent Document 10: Japanese Patent Application Laid-Open No. 56-25138), lead compounds (see, for example, Patent Document 11: Japanese Patent Application Laid-Open No. 57-176932), complexes of metals-such as copper, iron and zirconium (see, for example, Patent Document 12: Japanese Patent Application Laid-Open No. 57-183745), titanic acid esters (see, for example, Patent Document 13: Japanese Patent Application Laid-Open No. 58-185536), mixtures of a Lewis acid and a protonic acid (see, for example, Patent Document 14: Japanese Patent Application Laid-Open No. 60-173016), compounds of Sc, Mo, Mn, Bi, Te or the like (see, for example, Patent Document 15: Japanese Patent Application Laid-Open No. 1-265064), ferric acetate (see, for example, Patent Document 16: Japanese Patent Application Laid-Open No. 61-172852), and so on have been proposed. However, the main reason that the aromatic carbonate yield is low is not the reaction rate being slow but rather the equilibrium being biased markedly toward the original system, and hence improving the reaction rate through catalyst development cannot be said to be effective in improving the aromatic carbonate yield.
Regarding the reaction represented by above formula (E1), processes are known in which the by-produced alcohol is removed from the reaction system so as to shift the equilibrium toward the product system and thus promote the reaction. For example, batch reaction systems using a reactor provided with a distillation column are known (see, for example, Patent Document 2: Japanese Patent Application Laid-Open No. 56-123948, Patent Document 6: Japanese Patent Application Laid-Open No. 60-169444, Patent Document 7: Japanese Patent Application Laid-Open No. 60-169445, and Patent Document 14: Japanese Patent Application Laid-Open No. 60-173016). However, to make the aromatic carbonate yield high, operation must be carried out for a very long period of time, and moreover a large apparatus is required relative to the amount produced of the desired product; for such reasons, such a batch reaction system is not industrially practicable. As other processes in which the by-produced alcohol is removed from the reaction system, there have been proposed, for example, for the reaction between dimethyl carbonate and phenol, a process in which by-produced methanol is distilled off by azeotropy together with an azeotrope-forming agent (see, for example, Patent Document 17: Japanese Patent Application Laid-Open No. 54-48732 (corresponding to Patent Document 18: West German Patent Application Laid-Open No. 736063, and Patent Document 19: U.S. Pat. No. 4,252,737), Patent Document 20: Japanese Patent Application Laid-Open No. 61-291545), and a process in which the by-produced methanol is removed by being adsorbed onto a molecular sieve (see, for example, Patent Document 21: Japanese Patent Application Laid-Open No. 58-185536 (corresponding to Patent Document 22: U.S. Pat. No. 410,464)).
A process is also known in which aromatic carbonates are produced using a multi-stage distillation column so as to promote removal of the by-produced alcohol from the reaction system. In this process, a dialkyl carbonate and an aromatic hydroxy compound are continuously fed into a multi-stage distillation column, and reaction is carried out continuously in the distillation column, while continuously withdrawing by distillation a low boiling point component containing a by-produced alcohol from an upper portion of the column, and continuously withdrawing a component containing produced aromatic carbonates from a lower portion of the column (see, for example, Patent Document 23: Japanese Patent Application Laid-Open No. 4-224547).
Regarding the disproportionation reaction represented by above formula (E4), processes are known in which the by-produced dialkyl carbonate is removed from the reaction system so as to shift the equilibrium toward the product system and thus promote the reaction. Examples include a process in which an alkyl aryl carbonate is continuously fed into a multi-stage distillation column, and reaction is carried out continuously in the distillation column, while continuously withdrawing by distillation a low boiling point component containing the by-produced dialkyl carbonate from an upper portion of the column, and continuously withdrawing a component containing a produced diaryl carbonate from a lower portion of the column (see, for example, Patent Document 24: Japanese Patent Application Laid-Open No. 4-9358).
The reactions represented by above formulae (E1) to (E4) are generally promoted using a catalyst. The catalyst may be solid or liquid, and there have been reports regarding methods of feeding in the catalyst. For example, known methods include a method in which a titanium tetraalkoxide having alkoxy groups corresponding to the alkyl groups in a dialkyl carbonate used as a raw material is fed in, whereby contamination with by-produced material due to an alkyl alcohol by-produced from the catalyst is prevented (see, for example, Patent Document 25: Japanese Patent Application Laid-Open No. 2000-72721), a method in which, although it is known that the catalyst may precipitate out upon prolonged operation, a dialkyl carbonate, an aromatic hydroxy compound and a catalyst are continuously fed into a reactor, a by-produced alcohol is continuously withdrawn from a distillation column attached to the reactor, and aromatic carbonate containing an alkyl aryl carbonate, a diary carbonate, or a mixture thereof is withdrawn from the reactor, whereby clogging of the distillation column is prevented (see, for example, Patent Document 26: Japanese Patent Application Laid-Open No. 6-157410), and a method in which clogging is prevented by separating the feeding position for a catalyst from the feeding position for a dialkyl carbonate and an aromatic hydroxy compound (see, for example, Patent Document 27: Japanese Patent Application Laid-Open No. 2000-307400).
The catalyst used in such a reaction system is generally present dissolved in the reaction liquid under the reaction conditions, and moreover has a higher boiling point than that of the aromatic carbonates, and hence to obtain a high-purity aromatic carbonate from the liquid produced through the reaction, first a low boiling point component is removed from the reaction liquid, and then the diaryl carbonate in the high boiling point component is separated from the catalyst, thus purifying the diaryl carbonate. It is known that in this case the catalyst may be recovered as a high boiling point component, and may be recycled, or may have a deactivated component removed therefrom. An example of the process for separating out the catalyst is the process described in Patent Document 28 (Japanese Patent Application Laid-Open No. 9-169704). Moreover, a process in which separating out of the catalyst is carried out by using a catalyst having a low boiling point such as an alkylamine is also known (see, for example, Patent Document 29: Japanese Patent Application Laid-Open No. 2003-238487).
Although research has been carried out into processes for producing aromatic carbonates through reaction between a dialkyl carbonate and an aromatic hydroxy compound as described above, in most cases (not only the above examples), the dialkyl carbonate or the hydroxy compound is used in excess. A process in which one of these compounds is used in excess, and unreacted compound is recovered and reused has very poor energy efficiency. Furthermore, in addition to the equilibrium being unfavorable, in most of the publicly known production processes described above, dimethyl carbonate is used as the dialkyl carbonate. Dimethyl carbonate has a low boiling point of approximately 90° C., and moreover forms an azeotrope with methanol by-produced in the reaction. The azeotrope has a lower boiling point than dimethyl carbonate, and hence when removing the by-produced methanol from the system, dimethyl carbonate is also removed at the same time. To increase the reaction ratio, it is thus necessary to feed in the dimethyl carbonate in an amount several times that consumed in the reaction, and hence the energy efficiency has been very poor.
Processes in which excessively used energy is recovered and reused are also known. For example, a process is known in which vapor produced in a reactor is brought back into contact with the reaction liquid indirectly so as to heat the reaction liquid and thus improve the energy efficiency (see, for example, Patent Document 30: Japanese Patent Application Laid-Open No. 2004-75577). However, because dimethyl carbonate is used as the dialkyl carbonate, there is the problem described above that to remove methanol by-produced in the reaction from the system, dimethyl carbonate must be removed from the system at the same time.
A process in which a dialkyl carbonate that does not form an azeotrope with the alcohol by-produced in the reaction is used is also known (see, for example, Patent Document 31: Japanese Patent Application Laid-Open No. 10-152455). In this process, there is a suitable difference in boiling point between the alcohol by-produced in the reaction and the dialkyl carbonate, and hence it is easy to separate off only the alcohol. However, because a batch reaction system is used, a large apparatus is required relative to the amount produced of the desired product, and hence the process is not industrially practicable. Moreover, a process in which a multi-stage distillation column is used, and a dialkyl carbonate that does not form an azeotrope with the alcohol by-produced in the reaction is used is also known (see, for example, Patent Document 27: Japanese Patent Application Laid-Open No. 2000-307400). In this process, under a condition of excess aromatic hydroxy compound, the catalyst is fed into only the bottom of the column, and hence the reaction proceeds only in the bottom of the column, and thus a problem of deposition of a catalyst-derived component on structures in the distillation column is resolved; however, because the reaction proceeds only in the bottom of the column, the multi-stage distillation column thereabove is provided only for carrying out separation by distillation on the compounds having a vapor pressure (the dialkyl carbonate, phenol, the alcohol by-produced in the reaction, etc.), and a large excess of the aromatic hydroxy compound such that the starting material (i.e. aliphatic carbonate)/reactant (i.e. aromatic hydroxy compound) molar ratio is 0.01 must be used to shift the equilibrium toward the product system in only the bottom of the column, and thus the aromatic carbonate yield obtained is low based on both the starting material (the aliphatic carbonate) and the reactant (the aromatic hydroxy compound).
In the above publicly known processes, there are problems that either the dialkyl carbonate or the aromatic hydroxy compound is used in excess, and excess energy is used, and furthermore there is a problem that the aromatic carbonate(s) comprising an alkyl aryl carbonate, a diaryl carbonate, or a mixture thereof cannot be obtained with a high yield.